Using Simulation Tools to Troubleshoot an LC Filter Design

Note: Ed Troy, owner of Aerospace
Consulting, LLC, was kind enough to offer a few of his articles for posting on RF Cafe. With more than 30 years
in the electronics communications design field, Ed has a lot of valuable knowledge to impart to us mortals ;-) This
first paper discusses a method for accommodating real-world stray capacitances in the simulation model for helping
to assure first-pass success on LC filter designs. He's a highly rated airplane pilot as well, which counts for
something in my book. Stay tuned for others...

Using Simulation Tools to Troubleshoot an LC Filter Design

Almost everyone uses simulation tools for designing RF, microwave, and high speed digital circuits today. It
is essential. But, very few engineers use those same tools for troubleshooting of circuits that are not working
correctly. It amazes me that many engineers and technicians still spend days tuning, tweaking, and adjusting RF
circuits that do not work properly on the bench. They could probably find, and fix, the problems in much less time
by firing up their simulation software. It is much faster to change values, both known and speculated, in a simulation
engine than it is to make the same changes on the test bench.

I was first introduced to the power of using simulation to troubleshoot circuits in the early 1980's. I was working
on a receiver that required a 70 MHz IF band pass filter. While many people may consider 70 MHz to be
VHF, or very high frequency, or even UHF, or ultra high frequency, I thought of it as a pretty low frequency, since
I was used to working at frequencies up to 18 GHz.

But, when my technician tested a 70 MHz band pass filter that I had designed, it did not work at all as
expected. In fact, the performance was so bad that I thought that perhaps someone had mixed up the capacitors or
inductors and thus installed the wrong values. So, I had the technician rebuild the circuit using components whose
values had been actually measured. The results were the same. The filter looked terrible.

This happened over 30 years ago, so I do not have the original design files or test results, but I can reproduce
them very easily in a circuit simulator. Lets say we need a 70 MHz band pass filter with a bandwidth of 5 MHz.
A few minutes with the Keysight (formerly Agilent) Genesys filter design tool produces the following circuit (figure
1) and response(figure 2).

Figure 1 - Ideal 70 MHz Band Pass Filter Design

Figure 2 - Filter Response of Filter in Figure 1

This looks pretty good! But, when my technician built it up, as I said, we got a response similar to what is
shown in figure 3.

Figure 3 - Measured Response of Circuit in Figure 1

OOPS! Something is obviously wrong. But, we measured the values and the inductors and capacitors were the correct
values. What could be wrong?

After looking at the situation for a while, I realized that there was a pad on the circuit board where L1 and
C1, L3 and C3, and L5 and C5 came together. Those pads were only about 0.1" x 0.1" in size, however. How could those
tiny pads matter? A quick calculation showed that those pads had a capacitance to ground of about 0.22 pF.
How did I come to that conclusion? By using the equation for a parallel plate capacitor. In our case, k, the relative
dielectric constant of our circuit board material was 3.3. E0 is a physical constant. The necessary calculations
are shown below, where the equation for the capacitance of a parallel plate capacitor is shown by the equation for
C0:

Since the pad is 100 mils square, or 0.1" x 0.1", and there are 0.0254 m/in

The board thickness was 33 mils, or .033 inches, or .033*.0254 meters

Again, so what? Well, let's re-simulate the circuit putting 0.2 pF capacitors at those points, as shown
in figure 4.

This is the same as the measured result. (Of course it is the same in this case since I used the simulator with
the 0.2 pF capacitors added to produce the "measured" result. But, trust me, the result we saw when we measured
the filter circuit in the early 1980's was very close to the "measured" result that I am using in this paper. I
no longer have access to the actual circuit board or the decades old measured data.) So, the small, 0.22 pF
stray capacitances of the pads where the series L and C components were soldered to the board apparently created
the terrible filter response.

So, now what? We have to have those pads. Without them, the circuit could not be built.

Let's redesign the filter with those pad capacitances as fixed. We will adjust the values of everything else
to work properly with the 0.22 pF stray capacitances of the board pads. This optimization results in the circuit
shown in figure 6 and the results are in figure 7.

Figure 6 - New Circuit. The 0.22 pF Capacitors Are Strays and Not Part of the Populated Parts

While not identical to the original circuit simulation, this represents a usable filter which we built up and
used. (Of course, any experienced engineer recognizes that the values in these simulations are ideal and not actual
values of capacitance and inductance that can be purchased. But, by using standard value inductors and capacitors
we were able to get adequate performance once we re-optimized the circuit given the 0.22 pF stray capacitances
due to the physical pads on the boards that were necessary to connect the series L and C capacitors.)

Of course, we could have spent days changing values on an actual circuit board to eventually arrive at the necessary
performance, but it was much faster (and thus less expensive) to use RF simulation software tools to do the troubleshooting
and to come up with a great design.

Aerospace Consulting was started in 1982 as a part-time business. The founder,
Ed Troy, had
just changed jobs from working for a company that made telemetry transmitters and other military communications
equipment to a company that produced electronic article surveillance equipment. While working at ICI Americas, Ed
had an opportunity to do some design consulting with a company in New York that made hardware related to anti-submarine
warfare. Additionally, Narco Avionics needed help in redesigning some DME hardware because the transistor used in
their high power pulse transmitter was no longer available and a new amplifier had to be designed and developed.
With permission from ICI America's management, Ed started consulting on a part-time basis for these companies that
were clearly not competitors of ICI's EAS business. At the same time, Ed, a pilot with commercial, multi-engine,
and instrument ratings, was brokering aircraft in his free time. So, given the work for 2 aerospace-related companies
and the fact that he was brokering aircraft, Aerospace Consulting seemed a natural name for the company. Today,
it might seem a bit of a misnomer since most of Aerospace Consulting's customers are not aerospace companies
(although some are), but the name has not been changed. In the 25 years since going
full-time, Aerospace Consulting has grown dramatically both in terms of capabilities as well as the types of businesses
served. Aerospace Consulting also works with companies, both large and small, that have plenty of RF, microwave,
and wireless facilities and employees, but they just need some extra help to get through an especially busy time.

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas
and reference material while performing my work as an RF system and circuit design engineer.
The Internet was still largely an unknown entity at the time and not much was available
in the form of WYSIWYG
...

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